Generally, a wheel of a vehicle is coupled to an axle through a hub. In this case, the wheel and hub are fastened by a bolt (hereinafter referred to as a hub bolt) and nut. When the nut is fixed and the hub bolt is rotated in the fastening direction, the wheel and hub are compressed, while a tension (axial force or tightening force) is applied to the hub bolt, and the hub bolt extends. In the assembling operation or the like, it is important to adequately control the tightening fore (axial force) of the hub bolt to maintain at a proper value. This is because of a risk that the hub and/or wheel is deformed or broken, when the fastening is not adequate and/or tightening is excessively applied.
Various methods have conventionally been known, as a method for controlling the tightening force (axial force) of a hub bolt. For example, methods have been well known that a tightening torque is set at a predetermined value using a torque wrench, or a skilled mechanic or the like taps each portion of a bolt fastening member using a micro hammer, and based on the tap sound, inspects a tightening state of the hub bolt. Further, an ultrasonic applying measurement tester has also been known which applies ultrasonic pulses to a tightened hub bolt from an end surface in the axial direction, obtains a length (elongation) of the hub bolt from the time required for the reflective wave to return, and using the bolt length, calculates the axial force of the hub bolt.
However, in the method of using the torque wrench, the use is predicated on the muscular strength of a person, and therefore, limited to small-size vehicles or the like. In this torque wrench method, since the tightening torque is set at a predetermined value and the bolt is merely tightened to the set value, it is not possible to examine and/or measure the axial force itself of the hub bolt after the bolt is tightened. Further, in the torque wrench method, an unstable factor of torque coefficients is considered as a defect.
Meanwhile, in the inspection method using the tap sound with a micro hammer, since decision is made by listening based on the skill and intuition without the rating, it is not possible to examine and/or measure a fastening state of the hub bolt with accuracy.
Further, in the method using the ultrasonic applying measurement tester, since the axial force of the hub bolt is obtained by calculating an elongation of the hub bolt targeted for the test using correlation, it is required to perform measurement at least twice, prior to and subsequent to bolt fastening, to determine the elongation of the hub bolt. Accordingly, there are limitations in time required for measurement and reduction in cost. Furthermore, the conventional ultrasonic applying measurement tester has a separate sensor portion and amplifier portion and thus a plurality of components, thereby is not suitable for operation requiring portability, nor does not have a power source, and therefore, can be used only in places with a commercial power source.